This invention relates to threshold signal determining circuits for use in scanning instruments.
Various scanning instruments are available for scanning a field which varies in opacity or reflectivity. Such instruments generally employ a light to electric transducer, which presents an output in accordance with the light received from the field as it is scanned. A photomultiplier amplifier, a vidicon tube or image dissector tube are examples of light to electric transducers used in scanning instruments. The output from the light to electric transducer is generally related to some threshold signal level when the instrument is required to provide an output involving an interpretation of the scan. For example, when the output from the light to electric transducer exceeds an established threshold signal level such output may indicate that the portion of the field giving rise to such output contains a condition being sought while any lesser output indicates the field portion then being scanned does not contain the condition being sought or visa versa.
Some instruments require manual adjustment of the threshold level by the operator with the results obtained checked against an operator count or operator measurement of the condition being examined.
It is desirable that any threshold level signal for a scanning instrument be established as quickly and accurately as possible without requiring any judgment or apparatus adjustment by the operator and that it be made for each field to be examined. In addition, it is desirable that the threshold level signal be established immediately prior to an operative scan of the field.
There are applications for scanning instruments which require the scan of the field presented on a petri plate on which paper discs are located. It is desirable that a threshold signal determining circuit for an instrument used for such purpose be arranged so that the threshold level signal will not be determined in any way by the output of the light to electric transducer produced when such paper discs are scanned.